The practice of bagging bales of silage in large tubes of plastic film has developed as a method of maintaining the nutritional value of these bales during storage. A common practice has been to hold the tubes folded in multiple layers on a steel drum and to push the bales through the drum to pull the tube off the drum to cover the bales. With the rigid drum, the drum and the tube that mounts on it must be significantly larger than that of the bales in order to accommodate varying bale sizes and shapes. The oversized tube admits excess air into the tube and leaves the loose tube subject to wind damage.
In prior apparatus designed by the applicant, as exemplified by U.S. Pat. No. 4,945,715, issued Aug. 7, 1990, and U.S. Pat. No. 5,469,693, issued Nov. 28, 1995, the bales pass through a collapsible tunnel carrying the tube of film. The collapsible tunnels allow the use of a bagging tube with a smaller size, more closely approximating that of bales, than does a fixed diameter drum type tunnel.
The smaller tube has a number of advantages, including less air inclusion with the silage, reduced wind damage because of the tighter fit and the reduced material cost of the thinner, smaller bags that can be used.
Another type of apparatus that has been used in an attempt to achieve the benefits of a close fitting tube is the stretch bagger that uses plural fingers spaced circumferentially around the bales to stretch the tube to a size larger than the bales and then to release the tube to shrink onto the bales as it leaves the fingers. One device of this type is described in Koskela et al U.S. Pat. No. 5,220,772, issued Jun. 22, 1993. Baggers of this type must produce a large amount of stretch in the tube, sufficient that a bale may pass through without contacting the flat tube segments between the fingers. This is further complicated by the use of a flat pan on the bottom to support the bale and protect the tube from contact with the bale. The tube does not stretch well across the pan, so that additional stretch is required around the rest of the tube. The requisite high stretch can produce high stress concentrations in the plastic tube where it rides over the fingers. This may in turn produce stretch marks in the tube, which are undesirable lines of weakness. It has also been found with some machines of this sort that the stress on the fingers can be sufficient that the fingers will bend, allowing the folded tube to slide off the fingers prematurely.
Some prior art baggers have conical tube supports, either tunnels or fingers, which may result in the tubes sliding off the supports during dilation. With a conical support, the tube can crowd rearwardly so that inner layers roll up to the rear on the outer layers, pinching the outer layers. When film is pulled out from under the point where it is pinched, it tears. To address these problems, some finger type machines have fingers with a slight outwards slope towards their terminating ends. These fingers are bent resiliently inwards when the tube is stretched. As the tube comes off the fingers, the pressure from the multiple layers of tube is reduced and the fingers spring outwards, increasing the localized stress on the plastic film towards the end of the tube.
With finger type machines, the bag is spaced further from the bale where it travels over the fingers than it is in the "flats" between the fingers. The bag actually adopts a polygonal shape on the fingers. This means that bag material being drawn over the fingers must travel a longer path to the bale than the bag material between the fingers. This requires additional longitudinal stress and additional longitudinal stretch of the bag material drawn over the fingers. This can result in longitudinal stretch marks in the bag.
To accommodate bales of different sizes and shapes, some finger type machines permit the fingers to be positioned at varying radial positions with respect to a nominal centre of the bagging station. While this will alter the size and shape of the tube at the bagging station, it has been found that when the tube is stretched, the tension is not uniform circumferentially around the tube. This requires the use of a heavier and more expensive tube to accommodate the high tension zones.
The present invention relates to certain improvements in bale bagging apparatus that address certain of the concerns with the prior baggers.